The invention relates to block copolymers comprising at least two hard blocks S1 and S2 made from vinylaromatic monomers and, between these, at least one random soft block B/S made from vinylaromatic monomers and from dienes, where the proportion of the hard blocks is above 40% by weight, based on the total block copolymer.
The invention further relates to a process for preparing the block copolymers, and also to their use.
U.S. Pat. No. 4,939,208 describes linear, transparent styrene-butadiene block copolymers of the structure S1xe2x80x94B1xe2x80x94B/Sxe2x80x94S2. The polymerisation of styrene and butadiene in the presence of a Lewis base, in particular tetrahydrofuran (randomizer), gives the random copolymer block B/S. The length of the B/S segment depends on the amount of Lewis base.
EP-A 0 654 488 describes polymodal coupled styrene-butadiene block copolymers. The blocks B/S contain a styrene gradient (tapered block). By adding polar compounds, such as tetrahydrofuran, as randomizers, the random proportion in the blocks can be increased.
Polymerisation of styrene and butadiene in the presence of small amounts of tetrahydrofuran as randomizer gives a high proportion of homopolybutadiene blocks and a tapered transition to the polystyrene block. If the amount of tetrahydrofuran is increased, this gives butadiene-styrene copolymer blocks with some degree of random character, but at the same time there is also a sharp increase in the relative proportion of 1,2 linkages in the polydiene (1,2-vinyl content). The high 1,2-vinyl content, however, impairs the thermal stability of the corresponding block copolymers and increases the glass transition temperature.
DE-A 19615533 describes an elastomeric styrene-butadiene block copolymer in which the relative proportion of 1,2 linkages in the polydiene is 15% and the proportion of the hard phase is from 1 to 40% by volume. The polymerization of the soft phase is undertaken in the presence of a soluble potassium salt.
The use of potassium alcoholates or potassium hydroxide and of organolithium polymerization initiators is described in U.S. Pat. No. 3,767,632, U.S. Pat. No. 3,872,177, U.S. Pat. No. 3,944,528 and by C. W. Wolfford et al. in J. Polym. Sci, Part. A-1, Vol. 7 (1969), pp. 461-469.
Random copolymerisation of styrene and butadiene in cyclohexane in the presence of soluble potassium salts is described by S. D. Smith, A. Ashraf in Polymer Preprints 34 (2), 672 (1993) and 35 (2), 466 (1994). The soluble potassium salts mentioned comprise potassium 2,3-dimethyl-3-pentanolate and potassium 3-ethyl-3-pentanolate.
It is an object of the present invention to provide a glass-clear impact-modified polystyrene which has a balanced toughness/stiffness ratio and does not have the abovementioned disadvantages. In particular, the impact-modified polystyrene should have high intrinsic thermal stability and reduced thixotropy. It should also be compatible with styrene polymers, so that transparent mixtures are obtained. Efficiency in impact-modification of styrene polymers, in particular standard polystyrene, should be increased.
We have found this object is achieved by means of block copolymers comprising at least two hard blocks S1 and S2 made from vinylaromatic monomers and, between these, at least one random soft block B/S made from vinylaromatic monomers and from dienes, where the proportion of the hard blocks is above 40% by weight, based on the total block copolymer. In preferred block copolymers, the 1,2-vinyl content in the soft block B/S is less than 20%.
For the purposes of the present invention, vinyl content is the relative proportion of 1,2 linkages of the diene units, based on the total of 1,2, 1,4-cis and 1,4-trans linkages. The 1,2-vinyl content of the soft blocks is preferably from 10 to 20%, in particular from 12 to 16%.
vinylaromatic monomers which may be used for the hard blocks S1 and S2 or else for the soft blocks B/S are styrene, xcex1-methylstyrene, p-methylstyrene, ethylstyrene, tert-butylstyrene, vinyltoluene or mixtures of these, preferably styrene.
Preferred dienes for the soft block B/S are butadiene, isoprene, 2,3-dimethylbutadiene, 1,3-pentadiene, 1,3-hexadienes or piperylene or mixtures of these, particularly preferably 1,3-butadiene.
It is preferable for the block copolymer to be composed exclusively of hard blocks S1 and S2 and also of at least one random soft block B/S, and not to contain any homopolydiene blocks B. Preferred block copolymers contain external hard blocks S1 and S2 having different block lengths. The molar mass of S1 is preferably from 5000 to 30000 g/mol, in particular from 10,000 to 20,000 g/mol. The molar mass of S2 is preferably above 35,000 g/mol. Preferred molar masses of S2 are from 50,000 to 150,000 g/mol.
Between the hard blocks S1 and S2 there may also be more than one random soft block B/S. Preference is given to at least 2 random soft blocks (B/S)1 and (B/S)2 having different proportions of vinylaromatic monomers and therefore different glass transition temperatures.
The block copolymers may have a linear or star-shaped structure.
The linear block copolymer used preferably has the structure S1xe2x80x94(B/S)1xe2x80x94(B/S)2xe2x80x94S2. The molar ratio of vinylaromatic monomer to diene S/B in the block (B/S)1 is preferably below 0.25 and in the block (B/S)2 is preferably from 0.5 to 2.
The star-shaped block copolymers used are preferably those which have a structure in which at least one arm of the star has a block sequence S1xe2x80x94(B/S) and one arm of the star has the block sequence S2xe2x80x94(B/S), or those in which at least one arm of the star has the block sequence S1xe2x80x94(B/S)xe2x80x94S3 and at least one arm of the star has the block sequence S2xe2x80x94(B/S)xe2x80x94S3. S3 here is another hard block made from the vinylaromatic monomers mentioned.
Particular preference is given to star-shaped block copolymers having structures in which the star has at least one branch having the block sequence S1xe2x80x94(B/S)1xe2x80x94(B/S)2 and at least one branch having the block sequence S2xe2x80x94(B/S)1xe2x80x94(B/S)2, or in which the star has at least one branch with the block sequence S1xe2x80x94(B/S)1xe2x80x94(B/S)2xe2x80x94S3 and at least one branch with the block sequence S2xe2x80x94(B/S)1xe2x80x94(B/S)2xe2x80x94S3. The molar ratio of vinylaromatic monomers to diene, S/B, is preferably in the range from 0.5 to 2 in the outer block (B/S)1 and preferably below 0.5 in the inner block (B/S)2. The higher content of vinylaromatic monomer in the outer random block (B/S)1 makes the block copolymer more ductile for unchanged total butadiene content, and this proves to be particularly advantageous in blends with standard polystyrene.
The star-shaped block copolymers with the additional inner block S3 have higher stiffness at comparable ductility. The block S3 therefore acts as a filler within the soft phase, without changing the ratio of hard phase to soft phase. The molar mass of the blocks S3 is generally substantially lower than that of the blocks S1 and S2. The molar mass of S3 is preferably in the range from 500 to 5 000 g/mol.
The novel block copolymers may, for example, be formed by sequential anionic polymerisation, where at least the polymerisation of the blocks (B/S) takes place in the presence of a randomizer. The presence of randomizers brings about the random distribution of the dienes and vinylaromatic units in the soft block (B/S). Suitable randomizers are donor solvents, such as ethers, e.g. tetrahydrofuran, or tert-amines, or soluble potassium salts. In the case of tetrahydrofuran, the amounts used for ideal random distribution are generally above 0.25 percent by volume, based on the solvent. At low concentrations, xe2x80x9ctaperedxe2x80x9d blocks are obtained with a gradient in comonomer makeup.
At the same time, the larger amounts specified of tetrahydrofuran are used, the proportion of 1,2 linkages of the diene units increases to from about 30 to 35%.
In contrast, if potassium salts are used there is only an insignificant increase in the 1,2-vinyl content in the soft blocks. The resultant block copolymers are therefore less susceptible to crosslinking and have lower glass transition temperature for the same butadiene content.
The potassium salt is generally used in molar deficiency, based on the anionic polymerisation initiator. The molar ratio selected of anionic polymerisation initiator to potassium salt is preferably from 10:1 to 100:1, particular preferably from 30:1 to 70:1. The potassium salt used should generally be soluble in the reaction medium. Examples of suitable potassium salts are potassium alcoholates, in particular a potassium alcoholate of a tertiary alcohol having at least 5 carbon atoms. Particular preference is given to use of potassium 2,2-dimethyl-1-propanolate, potassium 2-methylbutanolate (potassium tert. amylate), potassium 2,3-dimethyl-3-pentanolate, potassium 2-methylhexanolate, potassium 3,7-dimethyl-3-octanolate (potassium tetrahydrolinaloolate) or potassium 3-ethyl-3-pentanolate. The potassium alcoholates are obtainable, for example, by reacting elemental potassium, potassium/sodium alloy or potassium alkylates with the appropriate alcohols in an inert solvent.
It is useful for the potassium salt not to be added to the reaction mixture until the anionic polymerisation initiator has been added. In this way hydrolysis of the potassium salt by traces of protic contaminants can be avoided. The potassium salt is particularly preferably added just prior to polymerisation of the random soft block B/S.
Anionic polymerisation initiators which may be used are the usual mono-, bi- or multifunctional alkali metal alkyl compounds, alkali metal aryl compounds or alkali metal aralkyl compounds. It is advantageous to use organolithium compounds, such as ethyl-, propyl-, isopropyl-, n-butyl-, sec-butyl-, tert-butyl-, phenyl-, diphenylhexyl-, hexamethyldi-, butadienyl-, isoprenyl- or polystyryllithium, 1,4-dilithiobutane, 1,4-dilithio-2-butene or 1,4-dilithiobenzene. The amount of polymerisation initiator needed depends on the desired molar weight. It is generally from 0.001 to 5 mol %, based on the total amount of monomers.
A polymerization initiator is added at least twice during the preparation of the asymmetrical, star-shaped block copolymers. It is preferable for the vinylaromatic monomer Sa and the initiator I1 to be added simultaneously to the reactor and for the polymerization to be carried out to completion and then for vinylaromatic monomer Sb and initiator I2 to be added, again simultaneously. This method gives two xe2x80x9clivingxe2x80x9d polymer chains Saxe2x80x94Sbxe2x80x94I1 and Sbxe2x80x94I2 alongside one another, onto which the block (B/S)1 is added by way of joint addition of vinylaromatic monomer and dienes and, where appropriate, the block (B/S)2 is added by way of further joint addition of vinylaromatic monomer and dienes, and also, where appropriate, the block S3 is polymerized by way of further addition of vinylaromatic monomer Sc. The ratio of initiator I1 to initiator I2 determines the relative proportion of the respective branches of the star randomly distributed within each star-shaped block copolymer after the coupling process. The block S1 here is formed from the feeds of the vinylaromatic monomers Sa and Sb, and the blocks S2 and S3 solely by way of the feed Sb and, respectively, Sc. The molar initiator ratio I2/I1 is preferably in the range from 4/1 to 1/1, particularly preferably in the range from 3.5/1 to 1.5/1.
The polymerisation may be undertaken in the presence of a solvent. Suitable solvents are those aliphatic, cycloaliphatic or aromatic hydrocarbons which have from 4 to 12 carbon atoms and are usual for anionic polymerisation, such as pentane, hexane, heptane, cyclohexane, methylcyclohexane, iso-octane, benzene, alkylbenzenes, such as toluene, xylene or ethylbenzene, or decalin or appropriate mixtures. Preference is given to cyclohexane and methylcyclohexane.
The polymerisation may also be carried out without solvent in the presence of organyl metal compounds which slow down the rate of polymerisation, such as alkylmagnesium compounds, alkylaluminum compounds or alkyl zinc compounds.
Once the polymerisation has ended the living polymer chains may be capped using a chain terminator. Suitable chain terminators are protonating substances or Lewis acids, such as water, alcohols, aliphatic or aromatic carboxylic acids, or also inorganic acids, such as carbonic acid or boric acid.
Instead of adding a chain terminator once the polymerisation has ended, the living polymer chains may also be linked to give a star shape by polyfunctional capping agents, such as polyfunctional aldehydes, ketones, esters, anhydrides or epoxides. Symmetrical or asymmetrical star block copolymers whose arms may have the abovementioned block structures may be obtained here by coupling identical or different blocks. Asymmetrical star block copolymers are obtainable, for example, by preparing the individual arms of the star separately and/or by initiating more than once, for example initiating twice with the initiator divided in a ratio of from 2/1 to 10/1.
The novel block copolymers behave as glass-clear impact-modified polystyrene with a balanced toughness/stiffness ratio. Due to the random soft block B/S, the block copolymers of the invention are more thermally stable and ductile than corresponding block copolymers with a xe2x80x9ctaperedxe2x80x9d B/S block for unchanged diene content. The block copolymers prepared in the presence of a potassium salt and having low 1,2-vinyl content have particularly high intrinsic thermal stability.
The novel block copolymers also have good compatibility with other styrene polymers, and may therefore be processed to give transparent polymer mixtures. The novel block copolymers or polymer mixtures may be used for producing fibers, forms of moldings.